Ribosoms

Ribosomes are very small, spherical, membrane-less cell organelles consisting of ribosomal RNA and globular proteins with a diameter of 10 - 25 nm. They are found in all living cells. Ribosomes are not visible under the light microscope. Under the electron microscope they appear as small dots (see figure). There are several million ribosomes in metabolically active cells. Since ribonucleic acids bind basic dyes, the basophilicity of a cell indicates a high ribosome content. Ribosomes occur individually or are located one behind the other on messenger ribonucleic acids (messenger RNAs) as polyribosomes (= polysomes = ergosomes). They are also found embedded in the outer surface of the membranes of the rough endoplasmic reticulum and the outer nuclear membrane connected to it. Mitochondria also contain ribosomes.

Ribosomes are formed in the nucleoli and are composed of their two subunits. The subunits can also be found individually in the cytoplasm. In mammals and humans, these are a larger 60s subunit and a smaller 40s subunit. Both fuse to form the final 80s ribosome (s = band in ultracentrifugation where the particles are found). The 60s subunit consists of 50 different proteins of a "free" 5s rRNA (ribosomal ribonucleic acid) with 120 bases, a 160 base 5.8s rRNA bound to a third 28s rRNA, which has 4,800 bases. The proteins are made normally via protein synthesis, i.e. at ribosomes. The rRNA is made by transcription at the appropriate rRNA genes of the DNA. The assembly of the rRNA and protein molecules is done with the help of about 200 different enzymes. The 40s subunit is composed of 33 different proteins and a 1,900 base long 18s rRNA.

The actual task of ribosomes is translation, i.e. the conversion of the genetic information copied onto messenger ribonucleic acids (m-RNAs) into the sequence of amino acids thereby defined. The amino acids strung together form proteins. This process, also known as protein biosynthesis, takes place during the G1 phase of the cell division cycle, i.e. during the growth phase, which is part of the interphase.

Protein biosynthesis at ribosomes proceeds in 3 steps:

1. initiation: an m-RNA binds to the 40s subunit of a ribosome.

2. elongation: starting at start triplet or start codon, transfer ribonucleic acids (t-RNAs) with specifically bound amino acids attach to the ribosome one by one according to the copied genetic code on the m-RNA. With the help of the enzyme peptidyl transferase, the amino acids from which the protein is formed are linked together. In the process, the t-RNAs detach from the ribosome again after they have released their amino acids.

Termination = chain termination: As soon as the stop codon of the m-RNA is reached, the ribosome disintegrates into its subunits under GTP consumption and the finished protein detaches from the last t-RNA.

Since a large number of ribosomes are located one after the other on an m-RNA thread, several proteins can be formed synchronously.